Municipal solid waste (MSW) includes all solid materials disposed by municipalities. While some of this waste is recycled, the majority is typically dumped in landfills, where it decomposes over a period of decades or even centuries. It has long been recognized, however, that municipal solid waste contains organic materials that have energy content. If MSW is left untreated in landfills, this energy content drained slowly from the landfill by bacterial processes, which not only dissipate the concentrated energy, but also produce methane, which is a strong greenhouse gas. Some landfills have sought to collect this methane, which may be used for fuel; however, the conversion to methane takes place on long time scales, wastes much of the internal energy of the MSW, and is ineffective in recovering much of the available energy content of the MSW.
The earliest and most common method of recovering the energy from MSW is incineration. Incineration includes the combustion of MSW or refuse-derived fuel (RDF) to produce heat, which typically powers a turbine to produce electricity. Byproducts of incineration include fly ash, bottom ash, flue gases, and particulates. Fly ash and bottom ash are typically discarded in land fills. Flue gases and particulates can be scrubbed from the incineration flue stream prior to discharge into the atmosphere. However, effective removal of all harmful flue stream components can be prohibitively expensive.
Another method of recovering energy from MSW is pyrolysis, which involves heating the organic portions of the MSW, so that thermally unstable compounds are broken down and evaporate with other volatile components. These volatile components form a pyrolysis gas that includes tar, methane, aromatic hydrocarbons, steam, and carbon dioxide. The solid residue from pyrolysis process includes coke (residual carbon), which can then be burned or used for gasification. The byproducts of pyrolysis are often more stable and less toxic than incinerator ash.
A related method for recovering energy from MSW is gasification. This involves converting at least a fraction of the MSW into a syngas composed mainly of carbon monoxide carbon dioxide, and hydrogen. Gasification technology has existed for some two centuries. In the nineteenth century, the conversion of coal, often as a result of the coking process, into “town gas” provided a flammable mix of carbon monoxide (CO), methane (CH4) and hydrogen (H2) that was used for cooking, heating and lighting. During the twentieth century, more than a million biomass gasifiers produced CO and H2 to meet transportation and mobilization needs during World Wars I and II. With the discovery of vast quantities of domestic natural gas after World War II, coal and biomass gasification was no longer cost-competitive and disappeared as an industry. Modern researchers have struggled to make gasification systems cost effective.
Sometimes, gasification has been applied directly to the MSW; in other cases, the MSW is first pyrolyzed, then subject to a secondary gasification process. Gasification of MSW generally includes a mechanical processing step that removes recyclables and other materials that have no energy content. Then, the processed feedstock is heated in a gasifier in the presence of a gasification agent (including at least some oxygen and possibly steam). Gasifiers may have a number of configurations. For example, fixed-bed gasifiers place the feedstock in a fixed bed, and then contact it with a stream of a gasification agent in either a counter-current (“up draft”) or co-current (“down draft”) manner. Gasifiers may also use fluidized bed reactors.
Another method of treating MSW is treatment in the presence of oxygen with a high-temperature plasma. Such systems may convert the MSW to syngas, leaving vitrified wastes and metals as byproduct.
After gasification plasma treatment, the resulting syngas is typically scrubbed to remove at least some of the particulates, acid gases, and soluble compounds. The scrubbing of syngas is much easier than scrubbing the flue gas of an incinerator because of the much lower volume of gases. The scrubbed syngas may be used to generate electricity by combusting the syngas in a boiler and using the steam to produce electricity, or by sending it to a combustion turbine that produces electricity in single or combined cycle operations. Alternatively, the syngas may be fed into a plant for the creation of synthetic fuels such as hydrocarbons or alcohols. Synthetic fuels have the advantage that they may be transported long distances and used to generate energy in a variety of devices at locations other than the syngas processing plant.
To create hydrocarbons as synthetic fuels, a common method for converting syngas into synthetic fuels is the catalytic Fischer-Tropsch process. This process produces a mixture of hydrocarbons. Another possibility is to create ethanol, methanol, n-propanol, and n-butanol, which may be incorporated into automotive fuels for use in existing automobiles. There are several known methods for creating ethanol and other higher alcohols from MSW, including acid hydrolysis and various bio-processes. However, these methods can often provide less favorable economics than desired. There are other catalytic processes that produce large yields of ethanol or other alcohols. For example, U.S. patent application Ser. No. 12/166,212 (filed Jul. 1, 2008, and incorporated by reference herein) describes a catalytic process to convert a syngas into a mixture of primarily methanol and ethanol. Because of tight energy constraints and the low cost of fossil fuels, it has been difficult to make these types of processes economically feasible. Creating ethanol from MSW efficiently is a difficult process that involves carefully managing feedstock quality, energy, syngas composition, syngas quality, product purification, and intermediate products. What is needed is an integrated process capable of converting MSW into ethanol economically, with low waste that can be disposed of safely and economically, and with low consumption of energy.
With the advent of higher gasoline prices, the development of alternative and synthetic fuels continues to attract significant interest. Synthetic fuels have the advantage that they may be transported long distances and may be used to generate energy at a location other than the syngas processing plant. However, because of tight energy constraints and the historical low cost of fossil fuels, it has been difficult to make synthetic fuel processes economically feasible. For example, creating ethanol and other alcohols from syngas efficiently is a difficult process that involves carefully managing energy, syngas, and other intermediate products. What is needed is a process capable of converting the waste materials such as MSW to syngas, and then syngas into alcohols economically and efficiently.